Embodiments of the invention relate generally to a balun for use with radio frequency (RF) coils and, more specifically, to a non-cylindrical cable balun design having reduced space requirements and that provides for a lighter-weight RF coil.
MRI uses radio frequency pulses and magnetic field gradients applied to a subject in a strong homogenous magnetic field to produce viewable images. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it, in random order, at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR RF receiver coils receive the emitted electromagnetic signals emanating from the patient and use the acquired signals for image reconstruction. Before image reconstruction occurs, the electromagnetic signals received by the receiver coil elements are conditioned by a balun. The balun helps to improve performance of the receiver coils by providing a high impedance path for common-mode currents in the electromagnetic signal and a low impedance path for differential currents therein.
While conventional baluns are typically sufficient for their intended purpose, there are certain environments and/or implementations where the typical balun construction may be less than ideal. As one example, in some instances, the available space around certain RF surface coils may be constrained, such that that the profile of a conventional coaxial cable balun structure in the anterior/posterior (AP) direction might be overly obtrusive and cumbersome. For example, the available space in the expanded polypropylene (EPP) foam of a head-neck unit (HNU) coil for accommodating the RF coil structure and associated coaxial cable baluns may be quite limited. As another example, it may be desirable for an RF surface coil structure to be as lightweight as is feasible—such as when the RF surface coil is an anterior array (AA) coil positioned on top of a patient.
As still another example of an environment/implementation where the typical balun construction may be less than ideal, it is recognized that RF surface coils are often used in a hybrid or combination MRI system, where MR image data is acquired in conjunction with positron emission tomography (PET) image data. In combination PET-MRI systems, it is desirable to have minimum mass in the region of the PET detector array in order to provide for optimum image acquisition. That is, while in a stand-alone MR system the structure and mass of components within the bore has no effect on image acquisition and image quality, such is not the case in a PET-MRI system—as the mass in the PET detector region attenuates gamma rays, which reduces PET signal to the detectors and degrades image quality (IQ). Toward this end, it is recognized that the use of a conventional coaxial cable balun structure in an AA coil that is positioned in the region of the PET detector array can lead to a 9% PET peak single loss, which is undesirable and can negatively affect image quality.
It would therefore be desirable to provide a cable balun design that provides a lower profile inside RF coils and provides for a lighter RF coil structure. It would also be desirable for the cable balun design to provide less PET attenuation than a conventional coaxial cable balun.